The present invention relates in general to an automatic brake hold wherein application of the brakes can be maintained automatically after a vehicle operator brakes the vehicle to a stop, and, more specifically, to the manner of terminating an individual brake autohold event without canceling the brake hold function itself.
An automatic braking feature for motor vehicles has been introduced known by the terms automatic vehicle hold (AVH), automatic brake hold, or brake autohold. A driver-controlled switch determines whether the feature is active. When active, a controller monitors vehicle movement. When the vehicle brakes to a stop (with the feature active) and the driver releases the brake pedal, brake pressure is automatically held to keep the vehicle at a stop. When the driver takes action to resume motion (e.g., pressing the accelerator pedal), the brake torque is released so that full control is restored to the driver. Driver fatigue is reduced since the need for continued pressure on the brake pedal during a long stop is eliminated. The typical method used for releasing the held brake pressure is to have the driver press on the accelerator pedal. During most situations, the transmission selector remains in the Drive position. A preferred type of operation of brake autohold when the transmission selector is in the Reverse position is disclosed in U.S. application Ser. No. 14/527,949, filed Oct. 30, 2014, entitled “Automatic Brake Hold With Low Speed Maneuverability,” incorporated herein by reference.
A typical braking system (such as an antilock braking system, or ABS) may include hydraulic and/or electronically controlled actuators to supply brake pressure to selected vehicle wheels. A hydraulic circuit converting movement of the brake pedal into a brake pressure is easily configured to operate automatically for providing the brake autohold feature. Electric actuators are particularly useful in connection with implementing a parking brake function since they can be constructed such that no energy is required to maintain brake pressure once the actuator becomes fully engaged.
When both types of actuators are present and when a brake autohold event continues for a long period of time, a brake controller may automatically switch from hydraulic actuation to electric actuation in order to reduce energy use. This may be especially beneficial in a vehicle having an engine auto stop/start function, wherein the internal combustion engine may be briefly shut down (by stopping combustion) during a stop of reduce use of fuel, since the engine cannot then supply energy to support the brake actuation.
In vehicles with automatic transmissions, when the vehicle is at a standstill with the brake applied the engine continues to operate at idle (unless there is an engine auto stop event) and a small amount of torque is applied to the transmission. Without brake autohold, when the driver releases the brake pedal then the vehicle is able to creep at a slow speed in response to the idle torque. Creeping is very useful to the driver in certain situations. In particular, the vehicle can be moved slightly without having to operate the accelerator pedal (i.e., the foot can remain positioned at the brake pedal).
When a conventional brake autohold system is engaged, it may be more difficult to utilize the creep torque since the typical manner of ending the brake autohold event is to press down on the accelerator pedal. Movement of the accelerator pedal simultaneously causes an increase in the engine throttle so that addition engine torque is applied to the transmission. Thus, the vehicle may accelerate faster or less smoothly than compared with a conventional brake release. The driver could press the Brake Autohold selector switch on the instrument panel to end the brake autohold event, but that may be undesirable because this also results in the Brake Auto Hold Mode being turned off, so the selector switch would have to be pressed again in order to obtain an automatic brake hold during the next vehicle stop.